Flux pinning

If the entire body is superconducting, the Meissner fraction will be reduced due to flux pinning (40). How can one determine whether a low Meissner fraction is due to flux pinning and/or due to a sample problem like superconducting "shells" One approach... 

Figure 10 Comparison of partial magnetization hysteresis loops at 30 and 77 K for a superconducting T Ca j Ba2Cu20o single crystal (Tc is 105 K). The lack of hysteresis at 77 K is due to weak flux pinning.

Figure 12 Ratio of flux creep rate S(T) to the magnetization at 1 second MQ(T) versus temperature. S and MQ are obtained by fitting data such as those in Figure 11 using Eq. (13). The slope of the dashed line corresponds to an effective flux pinning potential U = 83 meV according to a simple thermally activated flux creep model which yields Eq. (14).

In the weak magnetic field region (0.6 T), the experimental results are not in agreement with the intrinsic pinning model. In this case, the flux pinning at the film surface becomes more dominant than the intrinsic pinning by the ab planes. 

Frozen current vortices (FCV) in superconductors can serve as essential elements of magnetic recording and as a convenient subject for investigations magnetic flux pinning at a transport current / across the superconductor, annihilation of two FCV with opposite magnetic flux, quantum behavior of frozen magnetic flux in the FCV, collective flow of FCV under the influence of the transport current I (Fig.5a). 

Johansen, T.H. (2000) Flux-pinning-induced stress and magnetostriction in bulk superconductors, Supercond. Sci. Technol. 13, R. 121-37. 

Further examples of positron study of defects in HTSC are studies carried out to understand the nature of flux-pinning defects that lead to an increase in critical-current density on neutron-irradiated Y 1 2 3. Experiments [59] on positron lifetime and critical-current density measurements on various neutron-irradiated samples of Y 1 2 3 indicate that the critical current density is correlated with the micro-void density, as obtained from the analysis of positron lifetime measurements. Investigation of defects in other HTSC superconductors, such as La-Sr-Ca-Cu-0 [60], Bi-Sr-Ca-Cu-0 [49], and Nd-Ce-Cu-O [52], have also been carried out. 

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